Default to Jacobi preconditioner for mixed problems

PETSc's MatNest objects don't support ilu preconditioning.  In fact,
apart from physics-based block preconditioning, the only allowed
preconditioner is Jacobi.  If we encounter a mixed system where the user
has not supplied a preconditioner, default to Jacobi so that they don't
see a horrible PETSc error.
Merge pull request #84 from firedrakeproject/mixed-pc-type

Default to Jacobi preconditioner for mixed problems.